Thermostructural composite materials are known for their good mechanical properties and for their ability to conserve these properties at high temperature. They comprise carbon/carbon (C/C) composite materials constituted by carbon fiber reinforcement densified by a carbon matrix, and ceramic matrix composite (CMC) material formed by reinforcement of refractory fibers (carbon or ceramic) densified by a matrix that is ceramic, at least in part. Examples of CMCs are C/SiC composites (carbon fiber reinforcement and silicon carbide matrix), C/C—SiC composites (carbon fiber reinforcement and a matrix comprising a carbon phase, generally close to the fibers, and a silicon carbide phase), and SiC/SiC composites (both reinforcing fibers and matrix made of silicon carbide). An interphase layer may be interposed between the reinforcing fibers and the matrix in order to improve the mechanical behavior of the material.
The usual methods of obtaining parts made of thermostructural composite material use the liquid process or the gas process.
The liquid process consists in making a fiber preform having substantially the shape of a part that is to be obtained, and that is to constitute the reinforcement of the composite material, and in impregnating said preform with a liquid composition containing a precursor of the matrix material. As a general rule, the precursor is in the form of a polymer, such as a resin, possibly diluted in a solvent. The precursor is transformed into a refractory phase by heat treatment, after eliminating the solvent, if any, and after cross-linking the polymer. A plurality of successive impregnation cycles can be performed in order to reach the desired degree of densification. By way of example, liquid precursors of carbon may be resins having a relatively high coke content, such as phenolic resins, whereas liquid precursors of ceramic, in particular precursors of SiC, may be resins of the polycarbosilane (PCS) or the polytitanocarbosilane (PTCS) or the polysilazane (PSZ) type.
The gas process consists in chemical vapor infiltration. The fiber preform corresponding to the part to be made is placed in an oven into which a reaction gas phase is admitted. The pressure and the temperature that exist inside the oven and the composition of gas phase are selected in such a manner as to allow the gas to diffuse within the pores of the preform so as to form the matrix therein by depositing a solid material in contact with the fibers, said material resulting from a component of the gas decomposing or from a reaction between a plurality of components. For example, gaseous precursors of carbon may be hydrocarbons that give carbon by cracking, e.g. methane, and a gaseous preform of ceramic, in particular of SiC, may be methyltrichlorosilane (MTS) giving SiC by decomposing the MTS (possibly in the presence of hydrogen).
There also exist combined methods using both the liquid and the gas processes.
Because of their properties, such thermostructural composite materials find applications in a variety of fields, whenever there is a need for parts that are to be subjected to high levels of thermomechanical stress, for example in aviation, in space, or in the nuclear industry.
Nevertheless, whatever the method of densification that is used, parts made of thermostructural composite material always present internal porosity that is open, i.e. in communication with the outside of the part. The porosity stems from the inevitably incomplete nature of the densification of fiber preforms. It leads to the presence of pores and/or cracks of greater or smaller dimensions that communicate with one another. As a result, parts made of thermostructural composite material are not impervious, which means, in particular, that they cannot be used directly for making walls that are cooled by a circulating fluid, for example wall elements for a rocket thruster nozzle, or combustion chamber wall elements for a gas turbine, or indeed wall elements for a plasma confinement chamber in a nuclear fusion reactor.
Treatments exist for parts made of thermostructural composite material, seeking to close the pores present in the material. By way of example, U.S. Pat. No. 4,275,095 describes a method of manufacturing a composite material part in which carbon fiber reinforcement consolidated by a carbon matrix is impregnated with molten silicon which reacts with the carbon present in the material so as to form silicon carbide. The material constituted in that way is still porous, so the part is coated in a layer of silicon carbide for closing the pores in its surface. However, composite material parts made in that way are only relatively impervious and they are suitable only for protecting the surface of the part against oxidation without conferring the part with a degree of imperviousness that would enable it to be put into contact with a fluid without any risk of leakage. Forming silicon carbide around fibers by causing the carbon to react with molten silicon inevitably leads to an increase in volume (in the range 10% to 20%), and that generates stresses which lead to cracks in the material. Consequently, in addition to the fact that the resulting material remains porous after the silicon carbide has been formed, thus requiring an additional deposit of silicon carbide on its surface, the material also presents cracks which mean that it is not possible to guarantee a high degree of imperviousness, in particular because of the mechanical and/or thermal stresses to which the parts might be subjected.
U.S. Pat. No. 4,766,013 describes another method of manufacture in which silicon carbide is deposited directly on reinforcing fibers by chemical vapor infiltration. Nevertheless, chemical vapor infiltration of silicon carbide, even when repeated with machining being performed between two infiltration operations so as to open up the pores at the surface, still does not suffice to obtain a part without any residual porosity. The part is thus not impervious at this stage of manufacture, and requires an additional deposit of silicon carbide to fill in the pores at the surface of the part, such that any damage to this surface coating compromises the imperviousness of the part.